In this project the purported beneficial effects of lactic acid bacteria, commonly used in milk fermentation, in suppressing colon tumor incidence will be tested. Specifically, the hypothesis that dietary supplements of Streptococcus thermophilus and Lactobacillus acidophilus suppress the fecal mutagenic activity in humans will be tested. In parallel, the effect of these supplements on fecal bile acid and neutral sterol profile and on the fecal bacterial beta-glucuronidase activity will be studied. About 100 healthy volunteers will be screened for fecal mutagenic activity. Thirty individuals showing fecal mutagenic ratio of > 3.0 will be randomly divided into two groups of 15 each. Each group will follow a sequence of consuming: a control supplement of 4 weeks followed by lactic culture supplement (S. thermophilus or L. acidophilus) at 1010 live cells/day/subject for the next 4 weeks. At the end of each supplement 48- hour fecal samples will be collected and assayed for mutagenic activity, bile acids, neutral sterols and bacterial beta-glucuronidase activity. Also, a 4-day dietary record will be kept at the end of each 4-week period. At the end of 8-week sequence, the two groups will be switched with respect to the lactic culture supplement and another 8-week supplementation experiment will be conducted. During the experimentation, the volunteers will be instructed not to change their dietary habits drastically. Paired t test and analyses of variance will be conducted to test statistical significance. One Ph.D. level graduate student, 1 M.S. level graduate student and 1 undergraduate student will be trained in this area.